1. Field of the Invention
The invention relates to an image coding technique used in compression, transmission or the like of an image, and in particular, to an image coding technique capable of optimally encoding a mixed image in which an artificial image and a natural image are mixed.
2. Description of the Related Art
A mixed image is an image in which a natural image such as a photograph and an artificial image such as a letter, graphics or illustrations coexist. As techniques for non-reversible compression of a mixed image, there have conventionally been widely employed various kinds of coding schemes such as JPEG, GIF, and TIFF.
An original image to be encoded is different in nature of a pattern therein according to whether the original image is a natural image or an artificial image. In a natural image, there is a large correlation between pixels and a continuous change in pixel value. On the other hand, an artificial image has a nature that not only many of intervals in which pixel values are continuous in a similar way, but also has many of sharp edges. With such a difference in nature of a pattern between the images based on attributes thereof, all kinds of mixed images have had difficulty in optimally encoding the mixed image by means of a conventional image coding scheme described above.
For example, in a case where a mixed image is encoded with JPEG, mosquito noise is generated in profiles of a letter or graphics in an artificial image portion, leading to a great visual degradation. This is because JPEG concentrates spectra in a low band, while deleting high band information by means of discrete cosine transformation (hereinafter referred as [DCT]). In a case where a mixed imaged is encoded with a scheme such as GIF or TIFF, a coding efficiency for a natural image is lower as compared with JPEG, though both schemes are suited for compression of an artificial image.
Hence, as a technique encoding such a mixed image optimally, a proposal has been made on a coding technique in which a mixed image is separated into two regions, one of which is a discrete gray scale region mainly constituted of an artificial image and the other of which is a continuous gray scale region mainly constituted of a natural image to encode the regions using run length coding and JPEG, respectively, based on characteristics of the regions (see Takeshi MOGI, “Compression of Natural and Artificial Mixed Image Based on Region Separation” The Institute of Electronics, Information and Communication Engineers, transaction of The Institute of Electronics, Information and Communication Engineers D-II, Vol. J82-D-II, No. 7, p. 1150-1160, July 1999).
In the image coding scheme separating a mixed image into two regions: a discrete gray scale region mainly constituted of an artificial image and a continuous gray scale region mainly constituted of a natural image, however, a computational volume is great in operation of region division of an image, which makes it unsuitable for a high speed image compression.
If parallel processing is implemented on a hardware, a high speed processing is enabled. In this case, however, a circuit scale is tremendously large.